Aromatic carboxylic acids are raw materials for a wide variety of manufactured articles. For example, terephthalic acid (TA) is manufactured world-wide in amounts exceeding 50 million metric tons per year. A single manufacturing plant typically can produce 100,000 to 1,250,000 metric tons of TA per year. Purified TA (PTA) is used, for example, to prepare polyethylene terephthalate (PET), a raw material for manufacturing polyester fibers for textile applications and polyester film and resin for packaging and container applications.
Crude forms of aromatic carboxylic acid product can be produced by the high-pressure exothermic oxidation of a suitable aromatic feedstock compound in a liquid-phase reaction. A source of molecular oxygen is used as the oxidant, and the reaction is catalyzed by one or more catalyst compounds. For example, crude TA (referred to herein as CTA) is produced during the initial oxidation of paraxylene (pX) and contains a predetermined amount of oxidation reaction intermediates and by-products. These oxidation reaction intermediates and by-products are then removed in subsequent purification steps to produce PTA.
In one well known process for producing PTA, the CTA formed from the oxidation of pX includes impurities such as 4-carboxybenzaldehyde (referred to herein as 4-CBA). The CTA is purified by chemically reducing the 4-CBA to p-toluic acid and washing the resulting p-toluic acid from the product to provide PTA. Processes for the production of PTA are disclosed, for example, in U.S. Pat. Nos. 5,723,656, 7,935,844, 7,935,845, and 8,173,834.
The reaction in which an aromatic reactant is initially oxidized to produce a first crude aromatic carboxylic reaction product is conducted under conditions that inadvertently consume some of the reactant materials, a process referred to generally herein as burning. Burning can also take place as the crude aromatic reaction product produced in the primary oxidation step is further purified to produce a purified aromatic carboxylic acid product (for example, PTA). However, most of the burning in the process for producing the purified aromatic carboxylic acid occurs during the initial oxidation step. Burning represents one of the primary variable costs of the process for making a purified aromatic carboxylic acid.